In r39368, the way this variable works was changed.
Before, it allowed every user to add those groups to themselves, now it can be filtered on a per-group basis.

The new syntax is $wgGroupsAddToSelf['group1'] = array( 'group2', 'group3' ); which allows any user in the group "group1" to add the groups "group2" and "group3" to themselves.
The old syntax will still work for allowing every user to add groups to themselves, but it is recommended that you use $wgGroupsAddToSelf['user'] = array( 'group1', 'group2' ); instead.

You may use $wgGroupsAddToSelf['group'] = true; to allow users in that group to add every group to themselves.